Electrical testing apparatus



Dec. 6, 1927.

- G. A. CAMPBELL ELECTRICAL TESTING APPARATUS Fi ed Jan. 4. 1922 2 Sheets-Sheec 1 v INVENTOR iflz ATTORNEY Dec. 6, 1927, 1,651,440

G'. A. CAMPBELL ELECTRICAL TESTING APPARATUS Filed Jan. 4. 1922 2 Sheets-Sheet 2 -INVEN TOR 62 Qwwel Pasta Dec. 6, 1927.

UNITED STATES,

PATENT- OFFICE.

61036! A. CAMPBELL, OF UPPER ION'ICLAIB, NEW JERSEY, ASSIGNOB IO AMERI- CAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION 03 NEW YORK.

ELECTBICAL rns'rnm nrm'rus.

Application fled January 4, 1922. Serial No. mass-1.

ill

. wires and the superposed-phantom circuit.

Such measurements may be made by the means and method disclosed in the patent to Blackwell and Anderegg, No. 1,064,433, dated June '10, 1913, and that to Campbell, Blackwell and Colpitts, No. 1,167,677, dated January 11, 1916. The measurement of ca-.

pacity unbalance may be more readily made than the measurement of direct capacity or direct capacity difference, and furthermore,-

. the capacity unbalance measurement is satisfactory in the balanced splicing of duplex cables.

In the development work accurate measurements are more important than in commercial work, and consequently it is sometimes desirable to measure the difli'erences in capacity separately rather than the capacity unbalance as disclosed in the said patents. This is' especially true in development work on open wire. It should be noted that the capacity unbalance referred to above is the sum of two or more of these direct capacity differences. The direct capacity diiierence is also of particular interest in studymg 1ncircuits .in'to cable telephone circuits, since the induced" noise depends somewhat upon the direct capaeity difference between the two sides of a circuitand the sheath.

It is the object of thisinventionto provide a simple \Vhea't-stone bridge'network adapted to measure rapidly not only the direct capacity dilferences but also direct "conductance differences between two conductors and a third conductor or group of conductors or ground. I

ductive interference from power. or telegraph ductor or group of conductors, and Fig.2 1

shows a network for measuring conductance difference of the same conductor arrangement, the latter network being designed to balance the circuit with respect to conductance variations in order that a sharper bal ance may be obtained in the making of the capacity difierence measurements, and Fig. 3 shows the combination of the circuits shown in Figs. 1 and 2,.which is the practical form in which these circuits are used.

In Fig. 1, 1 and 2 represent two conductors, the difi'erence in capacity or conductance of which to a third conductor or group represented by 3, it is desired to measure.

Each of the circles 4 and 5 represents a group of conductors containing one-half of the other conductors in the system of which 1 and 2 form a part, the said groups'being conductively connected'with the conductors 2 and 1 respectively, of the pair whose capacity difference, for instance, is to be measured. The said other conductors of the system are divided into two approximately equal groups 4 and 5 so as to reduce the amount of capacity and conductance required in the bridge for obtainin a balance, and also, in certain cases, to' re uce trouble from external interference. The groups 3, 4 and 5 remain unchanged throughout the two measurements required to obtain the desired difference. It should be noted that in measuring the direct differences between two conductors of a system and a third conductor one of the grou s, as for example 4, is grounded. The dlrect 1 to conductor 3 is re resented by the; condenser C, and similar y, the direct capacity of conductor 2 to conductor 3 is represented by the condenser Cb. The direct capacity to conductor 3 of the group of conductors 5 connected with conductor 1 is represented by the condenser 0,, and similarly, the direct capacity of conductor.

.capacity to conductor 3 of the group 4 condenser C Two adjustable condensers are connected between conductor 3 and the said conductors of the system. Thus condenser A has one set of fixed plates connected with thegroup of conductors 5 and the other set of fixed plates connected with the group conductors 4. The movable plates of the said condenser are connected to conductor 3. In like manner in the adjustable condenser B, one set of fixed plates is connected to conductor 1 and the other set to conductor 2, and the movable plates of the said condenser are connected to conductor 3. Bridged across conductors 1 and 2 is a current indicating device such as a telephone receiver. Two accurately balanced ratio arms R and R, connected with conductors 1 and 2, re spectively, complete the Wheatstone bridge network. Connected between the junction point of the arms R and R and conductor 3, is a source of otential O.

- In the conductance balancing network shown in Fig. 2, the same numerals have been used to represent the parts common to both the capacity and the conductance measuring networks. In Fig. 2, G. and G represent, respectively, the direct conductance between conductor 1 and conductor 2 and conductor 3; and G; and Gre represent the direct conductance to conductor 3 of the groups 5 and 4 respectively. Two variable resistances A and B complete the network. The variable resistance A is inserted between each of the groups 5 and 4 and conductor 3, and the variable resistance B between conductors 1 and 2 and conductor 3. 1

The network represented by Fig. 2 is mostuseful when measuring the direct capacity differences in' open wire'circuits where it is necessary to provide means for balancing the direct conductance difierences between the conductors whose capacity difference is to be measured, in order to obtain silence in the tele hone receiver when the direct capacity di erences are balanced. In measurements on cable circuits, however, the conductance adjustments are usually unnecessary. It has been found desirable, however, to provide such a conductance adjustment network for use in measuring capacity'unbalances between very long cable circuits, such, for example, as submarine cable circuits, in order to insure greater accuracy of measurement. Fig. 3 shows the combination of the circuits shown in Figs. 1 and 2. In Fig. 3, the capacities and the conductances existing between conductor 3 and conductors 1 and 2 and also the halves 4 and5 of the remainder of the group of conductors are represented by dotted lines, for the reason that these ca acities and conductances are arts of the circuit to'be tested. The variale condensers A and B and the variable resistances A and B are represented by solid lines since these'form part of the test- In order to facilitate the difference between two cable conductors and a third conductor. As stated heretofore, the direct conductance difference in cable circuits is relatively small, and consequently may be neglected in measuring the direct capacity difference. of such conductors. Let it be assumed that we desire to measure the difference in direct capacity between the conductor 1 and-conductor 3, and between the conductor 2 and conductor 3,that is to say,the quantity (351Gb. A difierenceof potential is applied by the source 0 between the junction point of the ratio arms R and R and conductor 3. With condenser B in its balanced or zero position, the network is balanced by varyin the position of the movable plates of con enser A until a balanced condition is indicated by a mini mum tone or by silence in the tele hone receiver. The position of the mova le plates of condenser A, shown in Fig. 1, indicates that we have added capacity to the side of the network re resented by the capacities Ca and (3,, an

that we have subtractedcapacit from the side represented by the capacities Cb and 0,. As is well known to those familiar with adjustable plate condensers, the reading of'the scale isusually the capacity difference caused by the movement of the movable plates. In this case it is more convenient to arrange the scale to read one-half this dilference. Consequently, when the bridged network is balanced, the relation of the various'factors is represented by the equation figure and similarly, condenser C occupies the position of condenser C... The ad'ustable condenser A is allowed to remain in the position in which balance was obtained. Now when a difi'erence of potential is established by the source 0 between the junction point of the arms R and R and conductor 3, condenser B is adjusted until another balanced condition is established in the network, which will be indicated by condition is reac silence in the tele hone receiver. When this had, the relation is represented by the equation Substituting in Equation (2) the value of 2c .'given by Equation (1) we have O.Ct,=c

That is to say, the setting of condenser B,

when a balanced condition is established in the network, indicated the direct capacity difierence between conductor 1 and 3, and conductor 2 and 3. Y

In the measurement of direct capacity difference between-two conductors of open wireis done by means of the network shown in Fig. 2. In eflecting' a balance under this condition, condenser A is manipulated in conjunction with the variable resistance A and condenser B in conjunction with the.

variable resistance B',that is to sa-y condenser A is varied until the minimum tone is produced by the telephone receiver, and then the variable resistance A is adjusted so as to reduce the tone produced by the telephonereceiver, thereby establishing a second minimum point. Further adjustment of the condenser A is then made, and likewise further adjustment of the variable resistance A.' By alternately adjusting the condenser and the-resistance, the point of balance of both the conductance and the capacity networks will be reached. When this condition exists,

' the ca acity balance will be represented by in negation, in the manner heretofore descri E nation (11) --above, and the conductance ba ance wil be represented by Conductors 1 and 2 are-then interchanged ,and'th'e adjustable condenser B and the variable resistance B adjusted until a balanced condition is reached, as indicated by silence or minimum tone in the telephone receiver. When this condition exists, the direct .cafiacity diiferenee will be represented b quation- (2). above, and the conductance w ill rence will be condenser A and the are alternately When'this condition is Substituting in Equation (.5) the value of G; given by Equation (4), we obtain It will, therefore be apparent that'by means of the networks shown in'Figs.- l and 2, itis practicable to measure direct capacity and conductance differences existing between any two conductors and third conductor or groups of conductors from which the magnitude of the direct admittance may be readily computed.

In the foregoing description and. in the drawin we have set forth the method of measuring the direct capacity diflerence or the direct conductance difi'erence of two conductors to a third conductor or group. of conductors, which is the general case. 1 If it is desired to make similar measurements of two conductors and ground, the ground con nection is removed from group 4 and connected to point 3, and the conductor which formerly occupied position 3 is put in either group 4 or 5. The same procedure is then followed in making the measurements to ground as was pursued in measuring the vallOllS differences between two conductors and tween two conductors or between a conduc tor and ground. Thus, for example, suppose it is desired to measure the direct capacity C between conductor 1 and conductor 3. Conductor 2 would then be included in group 4,

and the capacit C would then be merged- 1n the capacity 5,

of group 4 to conductor 3.' Group 4 would be conductively connected wlth the point 2 of the bridge network, and also grounded as shown in Fig. 1. The bridge would then be balanced by adjusting relations would be represented by Conductor 1 is shifted to point 2 of the bridge and is conductivel connected to the group 4.; Group 5 is condixctively connected to point 1 of the bridge network. With condenser A left in the same position in which balance was previously obtained, condenser B. is. adjusted until'a balance is obtained for the new arrangement of. the". conductors. obtained the relation is represented by obtained from Equation (8) and we find that C. c, (10) which means that the reading of condenser B, when balance is obtained is the direct caacity between conductor 1 and conductor 3.

he direct conductance between conductors 1 and 3 ma be measured by the network of Fig. 2, mo ified in the same manner as was the network of Fig. 1 to make the direct ca-- pacity measurement between the same two conductors. V In making the direct capacity and the direct conductance measurements between conductor 1 and ground, point 3 of the network is grounded and the ground is removed from group 4. The conductor shown at 3 would be included in either grou 4 or 5.

In order to facilitate he 0 ration of a bridge embodying the princip e of this invention, suitable switching means" would of course be provided to change the. relative positions of conductors 1 and 2 with respect to the arms of the bridge, which operation has been fully described. The switching means for the capacity measuring circuit is shown schematically in Fig. 3. With the switch 6 in the position shown conductor 1 is connected with the arm R, with which the group 5 also is connected. And, similarly, conductor 2 is connected with the arm R with which the group 4 is connected. By throwing the switch upwards the positions of conductors 1 and 2 will be interchanged and they will be interconnected, that is, 1 will .be connected with R and 2 with R, the groups of conductors 4 and 5 remaining ermanently connected with the arms R an R.

Although this invention has been described as embodied in a particular formand arrangement of parts, it is to be understood that it is capable of embodiment in other forms and arrangements without departin from the spirit and scope of the appende claims.

What is claimed is:

1. In an electrical measuring system the combination with two conductors of a third conductor having direct capacity to each of said two conductors, two groups of conductors each connected with one of said two conductors and having direct capacity to said third conductor, an adjustable condenser differentially connected between said third conductor and said groups, a second ad'ustable condenser differentially connected tween said third conductor and said two conductors, ratio arms bridged across said two con-. ductors, and a source of testing voltage connected between the said ratio arms and the said third conductor. y

2. In an electrical measuring system the combination with-two conductors of a third conductor,-two groups of conductors each connected with one of said two conductors, a variable resistance connected difierentially between said third conductor and said two conductors, and a second variable resistance differentially connected between said third conductor and said groups of conductors, ratio. arms bridged across saidtwo conductors, and a source of testing voltage connected between said ratio arms and said third conductor.

3. In an electrical measurin system the combination with a direct capacity difference measuring bridge of a direct conductance difference measuring bridge.

- 4. Inan electrical measuring s stem for determining the direct capacity di erence between two conductors of a system and athird conductor, the combination of two capacities constituting two arms of a Wheatstone bridge, two other capacities symmetrically disposed with respect to the said first capacities and connected in parallel therewith, -balanced resistances constituting the other two arms of the said bridge, a source of testing potential connected etween midpoints of the resistance arms and the midpoint of the capacity arms, an indicating device connected between the junction points of the said resistance arms with the said capacity arms, a variable capacity differentially connected in parallel with the said first two capacities wherebv their difierence in magnitude may be balanced, and a second variable capacity differentially connected in parallel with the said second two capacities whereby their difference in magnitude may be balanced.

5. In an electrical measuring system for determining the direct capacity difierence between two conductors o asystem and a third conductor, the combination of two condensers constituting two arms of a Wheatstone bridge, two other. condensers symmetrically disposed with respect to the said first two condensers and connected in parallel therewith,'balanced resistances constituting the other two arms of the said bridge, a source of testing potential connected across two opposite corners of the said bridge, an indicating device connected across the other two opposite corners of the said bridge, a variable condenser connected difierentially in parallel with the said first two condensers, switching means to interchange in position the said two conductors, and a second variable condenser connected difierentially in.

parallel with the said other two condensers.

6. In an electrical measuring system for determining the direct capacity di erence between two conductors of a system and a third conductor, the combination of two" balanced ratioarms, a third arm consisting of a condenser,- a fourth arm consisting of a second condenser, athird condenser individual to' and inparallel with one of the said capacity arms, a fourth condenser individual to and nected in parallel with the said first two condensers, a second variable condenser dif ferentially connected in parallel with" the said last two condensers, a source of testing potential and means to indicate when the said brid e is in balanced condition.

7. A eatstone bridge network for measuring the difference between the direct capacities to ground of two of a grou of con ductors, comprising two ratio arms, t e inner ends of which are joined to constitute one corner of the said bridge and each outer end is connected with one of the said two conductors and also with one-half of the remainder of the said oup of conductors to constitute second and third corners of the said bridge, of which the ound is the fourth corner, a variable con enser connected dilferentially between the said two conductors and ground, a second variable condenser connected difie'rentially between' the said halves. of the said oup of conductors and ground, a source 0 alternating potential connected between one pair of opposlte corners of the said bridge, and. a current indicating instrument connected across the other pair of opposite corners.

8. A Wheatstone bridge network for measuring the difference between the direct capacity of two of a group of conductors to a third conductor, comprising two ratio arms, the inner ends of which are 'oined to constitute one corner of the said ridge and each tors to. constitute second an third corners of of opposite corners.

the said bridge of which the said third conductor is the fourth corner, a variable condenser connected difierentially between the said two conductors and the saidthird com ductor a second variable condenser connected am saidgroup of conductors and the saidthird conductor, a source of alternating potential ,connected between one pair of opposite corners of the said brid e, a currentrmdicating instrument connecte across'the other pair 9-. A Wheatstone bridge network for measuring the difierence between-the direct capacities and the-direct conductanccs of two of a group of conductors to a third conductor comprising tworatio arms, the inner on s being joined to constitute onecorner of e i the said brid e, and each outer end being connected wit one of the said two conductors, and also with one-half of the remainder of the said grou of conductors to constitute second and'thir corners of the said 'brid e, of which the said third conductor is e fourth corner,a variable condenser and a vaerentially between said halves of the v riable resistance, each coniiected difl'erentially between the said two conductors and the said third conductor, a second variable condenser and a second variable resistance, each connected differentially between the said halves of the said group of conductors and the said third conductor, a source of alternating potential connected between one pair of opposite corners of the said bridge,

ed across the otherpair of corners.

In testimony whereof, I have and a current indicating instrument connectname to this January, 1922.

ex eddn I specification 31i day 0% GEORGE a.- CAMPBELL. 

